Safer fetal test for genetic diseases on horizon

At the moment, testing unborn babies for genetic disorders carries an increased risk of miscarriage – but that could all be about to change

(Image: Gary Bistram/Rex Features)

It could soon be much easier and safer to determine whether an unborn baby has a genetic disorder.

Using just a drop of the mother’s blood, scientists hope to be able to determine if the fetus has a disorder such as cystic fibrosis or beta-thalassaemia. This avoids the need for the current invasive tests such as amniocentesis, which involves taking a sample of amniotic fluid surrounding the fetus and can increase the risk of miscarriage.

“We believe that the large proportion of invasive prenatal tests will one day be replaced,” says Dennis Lo of the Chinese University of Hong Kong, who led the research.

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Gene puzzle

The blood of a pregnant woman contains DNA from both herself and from her fetus. This genetic material can be removed from a blood sample and screened for disease genes.

However, as babies only develop diseases such as cystic fibrosis and beta-thalassaemia if they inherit two copies of the recessive gene responsible – one from each parent – the mere presence of the recessive gene in the mother’s blood does not indicate whether the fetus will develop the disease.

Finding the gene could just mean that the mother carries a copy of the gene, or that the fetus has inherited one, but not two, copies.

To get around this problem Lo’s team used a technique called digital polymerase chain reaction (dPCR) to count copies of the gene for beta-thalassaemia in blood samples from pregnant women.

Testing is still at a preliminary stage, but by noting ratios of recessive genes relative to normal copies of the gene, the researchers were able to work out if the fetus would develop the disease.

The team now aims to do larger trials and apply the same test to women carrying the gene for cystic fibrosis.

Lyn Chitty, who studies fetal medicine at University College London, and is particularly interested in using fetal DNA for prenatal diagnosis says&colon; “It’s another step in the right direction.”

More tests, more abortions?

“This [new] test saves those babies who would be incidentally aborted by invasive testing,” says Lo.

With the risk of miscarriage removed from prenatal tests, will more people eventually have the tests, leading to more abortions? Chitty says it isn’t that clear cut. “There may be more tests, but people don’t just have a test [for Down’s syndrome] to decide whether to have an abortion, sometimes it’s just to prepare themselves.”

She also points to figures released earlier this week by the Down’s Syndrome Association (DSA) in the UK that show that the number of babies born with Down’s syndrome has increased since 1989, despite the introduction of widespread screening.

John Smithies of the DSA attributes some of the increase to rising maternal age, but says the results of a DSA survey suggest that more people may also be actively deciding to raise a child with Down’s, perhaps because “life and society in general have improved”, with “family and friends offering more support”.

Sequenom, a San Diego company that collaborated with Lo on this research, is already planning to commercialise its non-invasive test for Down’s syndrome.

Journal reference&colon; Proceedings of the National Academies of Science, DOI&colon; 10.1073/pnas.0810373105